(a) Field of the Invention
The present invention relates to a zoom lens system and, more particularly, to a zoom lens system to be used with a home video camera or the like which has a large aperture and covers the range of focal lengths used frequently.
(b) Description of the Prior Art
There are known zoom lens systems comprising four lens groups in which two movable lens groups are arranged between fixed lens groups and those movable lens groups are moved independently of each other for the purpose of zooming.
Such known zoom lens systems for home video cameras generally have performances higher than those of image pickup tubes and tape decks and, therefore, they are satisfactory as far as the performance is concerned.
However, in case of known zoom lens systems for home video cameras, the number of lenses constituting the lens system is large, magnification is high and aperture is large. Consequently, the front lens diameter is large and overall length of the lens system is long. This means that it is difficult to make the camera as a whole small in size, light in weight and low in price.
Known zoom lens systems for home video cameras are disclosed, for example, in Japanese published unexamined patent applications Nos. 53718/82 and 27219/82. In case of the former zoom lens system out of the above, the aperture ratio is small, i.e., F/1.8 and, moreover, the overall length is long, i.e., 2f.sub.T (reference symbol f.sub.T represents the focal length of the lens system as a whole in the teleposition). In this zoom lens system, all lenses constituting the first lens group out of four lens groups are arranged as plastic lenses in order to make the lens system light in weight. However, the overall length is comparatively long as described in the above and, therefore, the lens system is not compact. Moreover, to make the lens system still lighter in weight and to reduce the cost of manufacture, it is necessary to increase the number of plastic lenses, and it is desirable to use plastic lenses also in other lens goups.
The latter zoom lens system employs a lens formed by fixing a thin plastic film with an aspherical surface onto a glass lens in order to improve the performance. However, in case of this zoom lens system, the aperture ratio is F/1.6 and zoom ratio is 3. Therefore, it cannot be said that it is a high performance zoom lens system.
Japanese published unexamined patent application No. 147133/81 also discloses a zoom lens system. Though this zoom lens system employs a lens with an aspherical surface, its aperture ratio is F/1.6 and zoom ratio is 3. Therefore, the performance of this zoom lens system is not so high.
Japanese published unexamined patent application No. 57018/81 also discloses a zoom lens system. This zoom lens system employs a glass lens having an aspherical surface and its zoom ratio is 8. However, its aperture ratio is F/1.8, and this is too small as the aperture ratio of zoom lens systems to be used with 2/3 inch image pickup tubes which are popular in these days.